1. Field of the Invention
The present invention relates to the field of controlling the engine idling speed at the desired speed by controlling the degree of opening of the valve in the air bypass passage connecting the pair passage upstream and downstream of a throttle valve. The desired engine idling speed is advantageously set so that both fuel economy and acceptable emission levels are achieved.
2. Prior Art
U.S Pat. No. 4,747,379 discloses an idle speed control device in which the closed-loop control and open-loop control together with the learning control are carried out to control the engine idle speed to the desired value In the closed-loop control mode of this system, the duty cycle for the idle speed control valve is updated at a predetermined time or at a fixed crank angle. The gain used in updating the duty cycle in closed-loop is also fixed This invention represents a method which employs fixed predetermined gain and fixed control valve signal update time However, because of the response delay of the vehicle system, the duty cycle update time and the closed-loop gain are critical for maintaining the stable idling speed If the duty cycle for the idle speed control valve is updated too frequently and/or the closed-loop gain is too large, overadjustment is likely to occur which causes the cycling of the speed of engine revolution. This undesirable engine speed cycling may also result in an engine stall. On the other hand, if the duty cycle is updated too slowly and/or the closed-loop gain is too small, the system may respond inadequately to the engine speed change so that an engine stall may occur when the engine speed is suddenly lowered to a large extent and a speed flare may happen when the load on the engine is greatly reduced. These problems as mentioned above can happen if the closed-loop gain and/or the update time are fixed as in the above-mentioned disclosure.
U.S Pat. No. 4,457,276 discloses an idling speed control system in which the target opening angle of the throttle valve to bring the idle speed towards the desired speed is calculated. The difference between the target throttle valve opening angle and the actual opening angle is used to obtain the duty cycle signal for the bypass passage control valve for the extra intake air. In this system, the desired throttle opening angle is the sum of the base opening angle for the target rpm, a first correction term, and a second correction term. The first correction term is the product of a constant and the engine idling speed deviation. The second correction term is in used when the idling speed is less than a predetermined limit, Nm. It is this second correction term that provides the extra air required to prevent the engine from stalling without causing overshoot. In the proposed system, the extra intake air is increased little by little if the engine speed is lowered to a small extent with respect to the desired idling speed, the extra amount of intake air is increased by a large amount when the actual idling speed of the engine drops significantly below the desired idling speed. The calculation is done once every 30 msec. The '276 disclosure represents a method which varies the control signal according to the idling speed deviation, but uses a fixed update time Thus, the above-mentioned problem is likely to occur. In addition, it only addresses the problem where the idling speed is significantly below the desired speed for stall prevention. The speed flare problem where the idling speed is significantly above the desired speed is not addressed by the '276 patent.
U.S. Pat. No. 4,557,234 discloses an idle speed control system which uses a simplified control device where a bypass passage is either fully opened or blocked. If the idle speed stays within the preset desired range, 630 rpm to 780 rpm, the state of the bypass passage is not changed. If the idle speed stays below 630 rpm for a predetermined period, i.e., C2&gt;A, A is a predetermined value (for example, 32), the bypass passage is opened to increase the engine speed. C2 is incremented by 1 every 32 msec. When the engine speed is lowered to 550 rpm or less, C2 is doubly increased by increments in order to shorten the time period for opening the bypass passage and thus provide a more responsive control. On the other hand, if the engine speed stays above 780 rpm for a predetermined period, i.e., C1&gt;B, B is a Predetermined value (for example, 48), the bypass passage is blocked to reduce the engine speed. C1 is incremented by 1 every 32 msec. Again, to provide a more responsive control, C1 is doubly increased by increments when the engine speed is higher than 950 rpm. Although the time period to update the state of the bypass passage is different for different speed range, it is not truly a function of engine speed deviation. In fact, only four time periods are defined for four different speed ranges. Therefore, the update time may not correspond closely to the desired for all engine speed. In the case of high idling speed, the idling speed will remain high for a long time because of the slow system response. In the case of low idling speed, the system may respond too slow such that the engine stalls.